A generic gas distributor is known from EP 0 687 749 A1. The gas distributor is located in a CVD reactor and forms the top of a process chamber, the bottom of which forms a substrate holder, on which one or more substrates are disposed in order to be coated, the materials that form the layer being introduced into the process chamber by means of the gas distributor. The gas distributor known from EP 687 749 A1 has a number of gas volumes disposed one above the other, which are respectively supplied with a different process gas. Each of the gas volumes has outlet channels, which open out in the bottom of the gas distributor. The two gas volumes lie one above the other and extend over the entire cross-sectional area of the gas distributor. The supply lines to the gas volumes are located at the periphery, so that the gas emerging from the gas distributor may have non-homogeneities.
Gas distributors of the generic type are used in the case of MOCVD. Metal Organic Chemical Vapor Deposition (MOCVD) is a widely used method for depositing single- or multi-component oxidic insulating layers (dielectrics), semiconductor layers, passivation layers or electrically conducting layers. For this purpose, a number of reactive gases or gaseous precursors are mixed, fed to a reaction chamber in order to deposit a layer on a heated substrate, and then pumped out from the reaction chamber. Among the reactors there are various geometric arrangements, for example horizontal and vertical reactors. In the case of horizontal reactors, the substrate surface is parallel to the direction of flow of the mixed precursors and reactive gases. In vertical reactors, the corresponding gas mixture impinges vertically on the substrate surface and flows off to the outer edges of the substrate before it leaves the reaction chamber. In general, rotation of the substrate can be used to increase the uniformity of the layer that is deposited.
In order to ensure homogeneous deposition on the substrate, thorough mixing of the various gaseous precursors or reactive gases must be ensured. In order to achieve this, there are methods by which the gas mixing is achieved at an early stage before introduction into the reaction chamber. This is suitable for precursors and reactive gases that are stable at the temperature and pressure in the gas distributor.
However, the precursors used are often very reactive and may thereby cause preliminary gas phase reactions. This leads to deposition on, and consequently progressive contamination of, gas-carrying parts upstream of the substrate, causes particle formation, and consequently particle coating of the substrate, changes the reaction chemistry at the substrate and reduces the efficiency of the growth process.
In the case of the aforementioned multi-chamber gas distributor, see also U.S. Pat. No. 5,871,586, the various gaseous components are supplied in separate chambers and fed directly to the substrate via a multiplicity of openings. The mixing only takes place in the region directly at the substrate. In the case of such a multi-chamber gas distributor, feed-through pipes lead from a first chamber into a gas distributor outlet and thereby cross through at least one other chamber. As a result, in the chambers there are narrow flow cross sections around the feed-through pipes. This leads to non-homogeneous flows and an increased pressure drop within a chamber. These problems increase with greater diameters of the gas distributor, since the number of feed-through pipes increases with the surface area. Furthermore, the production of the gas distributor becomes very much more complicated as the number of feed-through pipes increases, since each feed-through pipe has to be gastight at each separating wall of a chamber. Such a gas distributor is scarcely scalable and therefore cannot be produced or used in practice for the coating of relatively large substrates, e.g. 200 mm, 300 mm. After it has been produced, it is no longer possible in practice for such a gas distributor to be opened, for example for maintenance purposes.
In the case of some processes for oxidic insulating layers (dielectrics), passivation layers or electrically conducting layers, it has been found that this type of mixing does not lead to sufficiently homogeneous layers on the substrate. For some applications, the requirement for the non-homogeneity of the deposited layers on the substrate surface is, for example, <+1%.
Many gaseous metal-organic precursors are only stable as such within a small temperature range. (Metal-organic precursors may contain at least one metal atom and/or also at least one semiconductor/semimetal atom (such as for example Si, Ge)). At temperatures that are too low, condensation takes place; at temperatures that are too high, decomposition takes place even before mixing with other reactive gases. It is therefore necessary to keep the gas distributor at a homogeneous temperature.
On the basis of the generic prior art, an object of the invention is to improve the way in which a gas distributor operates.
The object is achieved by the invention specified in the claims, each individual one of the claims in principle representing independent solutions to achieve the object and it being possible for each claim to be combined with any other claim as an independent technical solution.
Claim 1 provides first and foremost that the gases are distributed in a radial direction in a first plane and then distributed in a circumferential direction in a second plane, lying under said first plane, before they leave the gas distributor through the outlet openings at the bottom of it.
Claim 2 provides first and foremost that each gas volume is formed by a number of pre-chambers, the pre-chambers lying in a common first plane, and a multiplicity of gas distributing chambers that are respectively associated with a gas volume being provided in a second plane, forming the bottom of the gas distributor, the pre-chambers and the gas distributing chambers of each gas volume being connected by connecting channels. Preferably, all the pre-chambers are disposed in a common first plane. In a development of the invention, it is provided that the pre-chambers that belong to one gas volume are at different radial distances from the center of the gas distributor. It is also provided that the pre-chambers belonging to one gas volume are disposed such that they are distributed in a circumferential direction. The pre-chambers of two different gas volumes may engage in one another in the manner of a comb. The prongs of the comb may in this case be continuations of each individual chamber that run in a radial direction. The gas distributing chambers may concentrically surround the center of the gas distributor. It is provided that a gas distributing chamber is connected to a number of pre-chambers. A pre-chamber may in turn also be connected to a number of gas distributing chambers. Preferably, the connecting channels between the individual chambers lie in a third plane, which is located between the first plane and the second plane. The invention provides a multi-chamber gas distributor wherein the gaseous precursors, which may comprise metals or semiconductors, and the reactive gases are introduced separately into a gas distributor. The gas distributor has a high degree of temperature homogeneity, in order to avoid condensation, decomposition and preliminary reactions of the precursors with the chemically reactive gases. In this respect, the smallest possible pressure drop on passing through the gas distributor is advantageous for the gaseous precursors. This is the case in particular when an evaporator is provided upstream of the gas distributor. With this evaporator, liquid or solid starting materials can be made to evaporate into process gases. In a preferred arrangement, the gas distributor is used in a CVD reactor. In this case, the gas distributor extends substantially parallel to a substrate holder. The substrate holder and the gas distributor then form the boundaries of a process chamber. In this case, the gas distributor may be located above, below or to the side of the substrate holder. Preferably, the gas distributor provides the upper boundary of the process chamber. The bottom of the gas distributor then forms the top of a process chamber. The bottom of the process chamber is the substrate holder. One or more substrates may be disposed on the substrate holder. The gas distributor has an overall appearance similar to a shower head. The process gases exit from the openings disposed at the bottom of the gas distributor, in order to react with one another in the gas phase or else on the substrate, a layer being deposited on the substrate. The mass flows of the gases can be set in such a way that the gases have a dwell time in the gas distributor of 10 ms to 16 ms. In this case, the individual chambers of the gas volumes are configured in such a way that this applies to overall gas flows of 300 to 1200 sccm. The pressure drop on flowing through the gas distributor is preferably <2.5 mbar, with a total flow of 1200 sccm. The temperature non-homogeneity along the gas flow path is preferably less than 10%. When the gaseous precursors exit from the gas outlet openings of the gas distributor, there is a standard deviation for the flow distribution of 0.3% to 0.9%. Nitrogen, hydrogen, helium and argon or some other noble gas or inert gas are to be preferred as carrier gases for the precursors. In a preferred configuration, gaseous precursors or starting materials that are liquid at room temperature or metal-organic starting materials are used. These are converted into the gas phase in special evaporation processes and are then fed to the gas distributor. There they enter the gas volume associated with them. The gas flow is split into a number of partial gas flows, with which the individual pre-chambers are supplied. Via the pre-chambers, the process gas then passes through the connecting channels into the gas distributing chamber circularly surrounding the center of the gas distributor. In the process, gases from different pre-chambers of the same gas volume enter one and the same gas distributing chamber. A reactive gas, for example O2, O3, NO2, H2O, NH3 or H2, is introduced into a second gas volume. This gas volume may have one or more pre-chambers. The pre-chamber or the pre-chambers are likewise connected via connecting channels to gas distributing chambers disposed concentrically around the center of the gas distributor. The gas distributing chambers belonging to the individual gas volumes may alternate in a radial direction. With the apparatus according to the invention, multi-component, oxidic insulating layers, dielectrics, passivation layers, semiconducting layers or electrically conducting layers or layer sequences are deposited on at least one substrate.